1. Field of the Invention
The present invention relates generally to vernier amplifiers.
2. Description of the Related Art
Digital-to-analog converters (DACs) convert digital input signals to corresponding analog output signals. In a typical DAC embodiment, an internal reference signal (e.g., a reference current) is mirrored to form a plurality of currents which flow through switches that respond to a digital command signal. The combined currents thus form an analog output current that corresponds to the digital input signal.
In a multiplying digital-to-analog converter (MDAC), an input signal Sin is received so as to modify the internal reference signal and thus the input signal Sin is converted to an analog output signal Soutwith a conversion gain that corresponds to the digital command signal.
The conversion gain of MDACs are often configured to provide coarse gain steps (e.g., 6 dB steps) which are then filled in with fine gain steps (e.g., 1 dB) from a vernier (that provides the MDAC""s reference signal) so that the combined system has a fine resolution over its entire operational range. MDACs, however, generally produce spurious signals in response to any direct-current (DC) offsets in their reference signal and it has been found that typical vernier structures are prone to generate such offsets in response to changes in their digital command signal.
The present invention is directed to verniers that substantially eliminate DC offset signals as they convert a differential input signal Sin to a differential output signal Sout with a conversion gain that corresponds to a digital command signal.
These goals are realized with various combinations of differential input pairs of transistors, pairs of reference transistors, pairs of subtraction transistors, differential pairs of reference transistors and differential pairs of subtraction transistors. These transistors are arranged to steer the current of one current source to other current sources so as to generate differential output currents with substantial elimination of offset currents.
The novel features of the invention are set forth with particularity in the appended claims. The invention will be best understood from the following description when read in conjunction with the accompanying drawings.